Melt overheating method for improved toughness and glass-forming ability of metallic glasses

ABSTRACT

A method of forming a bulk metallic glass is provided. The method includes overheating the alloy melt to a temperature above a threshold temperature, T tough , associated with the metallic glass demonstrating substantial improvement in toughness compared to the toughness demonstrated in the absence of overheating the melt above T liquidus , and another threshold temperature, T GFA , associated with the metallic glass demonstrating substantial improvement in glass-forming ability compared to the glass-forming ability demonstrated in the absence of overheating the melt above T liquidus . After overheating the alloy melt to above T tough  and T GFA , the melt may be cooled and equilibrated to an intermediate temperature below both T tough  and T GFA  but above T liquidus , and subsequently quenched at a high enough rate to form a bulk metallic glass.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Patent Application No. 61/755,177, entitled “Melt Overheating Method For Improved Toughness and Glass-Forming Ability of Metallic Glasses” filed on Jan. 22, 2013, which is incorporated herein by reference in its entirety.

FIELD

The present disclosure is directed to a method of overheating the melt of an alloy capable of forming metallic glass prior to quenching the melt in order to improve the glass-forming ability of the alloy and/or the toughness of the metallic glass.

BACKGROUND

Overheating the melt of alloys capable of forming metallic glass to temperatures sufficiently higher than the melting temperature is shown to influence certain kinetic properties of the liquid. Specifically, Lin et al. (U.S. Pat. No. 5,797,443) demonstrated that by overheating the melt of a bulk-solidifying Zr-based amorphous metal above a threshold temperature, which is sufficiently higher than the melting temperature, the degree to which the alloy can be undercooled to below the melting temperature by quenching increases. Lin et al. conjectured that by overheating the melt, certain oxide inclusions were dissolved into the melt and therefore could not serve as sites for heterogeneous nucleation of crystalline phases. The implication of a larger degree of undercooling is that the glass-forming ability of the alloy is enhanced. As such, the critical cooling rate (i.e. lowest cooling rate required to bypass crystallization of the alloy and form the amorphous phase) is decreased, while the critical casting thickness (i.e. largest lateral dimension of parts that can be formed with an amorphous phase) is increased. Lin et al. did not directly demonstrate that the critical casting thickness of the alloy increases with melt overheating, but concluded so by interpreting the undercooling results in the context of crystallization kinetics.

Lin et al. also discloses that after processing the melt at a temperature higher than T_(GFA), it is possible to cool and isothermally hold to an intermediate temperature between T_(GFA) and T_(liquidus) prior to quenching without substantially losing the gains in glass-forming ability attained by initially overheating to above T_(GFA). In other words, once the melt is heated to a temperature higher than T_(GFA), its capacity undergo deeper undercooling is maintained even if it is subsequently annealed at temperatures between T_(GFA) and T_(liquidus) prior to undercooling.

However, Lin et al. did not demonstrate, suggest, or imply that overheating the melt above some threshold temperature would have any influence on the mechanical properties of the amorphous metal, such as the fracture toughness.

BRIEF SUMMARY

The present disclosure provides methods of forming bulk metallic glasses or shaped metallic glass articles having higher toughness by overheating the alloy melt.

The disclosure is directed to a method of processing alloys into metallic glasses or metallic glass articles. The method includes melting an alloy by heating to a temperature above the liquidus temperature of the alloy, T_(liquidus). The method also includes overheating the alloy melt to a temperature above a threshold temperature, T_(tough), associated with the metallic glass (i.e. the alloy in an amorphous phase) demonstrating increased toughness compared to the toughness demonstrated by heating the alloy melt just above T_(liquidus). The method further includes quenching the alloy melt at a high enough rate to form metallic glasses or shaped metallic glass articles.

In another embodiment, the temperature of the overheated alloy melt is also above another threshold temperature, T_(GFA), associated with the alloy demonstrating increased glass-forming ability compared to the glass-forming ability demonstrated by heating the alloy melt just above T_(liquidus).

In yet another embodiment, T_(tough) is greater than T_(GFA).

In yet another embodiment, both T_(tough) and T_(GFA) are greater than T_(liquidus).

In yet another embodiment, the method also includes cooling the alloy melt following overheating to above T_(tough) and T_(GFA) to an intermediate temperature below T_(tough) and T_(GFA) but above T_(liquidus) and equilibrating the alloy melt at the intermediate temperature, and subsequently quenching the alloy melt at a high enough rate to form a metallic glass article.

In yet another embodiment, a method of forming a shaped metallic glass article is provided. The method includes melting a metallic glass forming alloy by heating the alloy to a temperature above the liquidus temperature of the alloy, T_(liquidus). The method also includes overheating the alloy melt to a temperature above both a threshold temperature, T_(tough), associated with the metallic glass demonstrating substantial improvement in toughness compared to the toughness demonstrated by heating the melt just above T_(liquidus), and another threshold temperature, T_(GFA), associated with the alloy demonstrating substantial improvement in glass-forming ability compared to the glass-forming ability demonstrated by heating the melt just above T_(liquidus). The method further includes simultaneously or subsequently quenching the alloy melt at a high enough rate to form a shaped metallic glass article.

In yet another embodiment, following overheating to above T_(tough) and T_(GFA), the method of forming a shaped metallic glass article includes cooling and equilibrating the alloy melt to an intermediate temperature below T_(tough) and T_(GFA) but above T_(liquidus). The method further includes simultaneously or subsequently quenching the alloy melt at a high enough rate to form a shaped metallic glass article.

In yet another embodiment, the metallic glass article having a lateral dimension of at least 0.5 mm made according to the present method is capable of undergoing macroscopic plastic deformation without fracturing catastrophically under a bending load.

In yet another embodiment, the alloy or metallic glass is Zr-based, Ti-based, Al-based, Mg-based, Ce-based, La-based, Y-based, Fe-based, Ni-based, Co-based, Cu-based, Au-based, Pd-based, or Pt-based.

In yet another embodiment, the alloy or metallic glass is represented by the following formula:

X_(100-a-b)Y_(a)Z_(b)  Eq. (1)

wherein:

X is Ni, Fe, Co, Pd, Pt, Au, Cu or combinations thereof;

Y is Cr, Mo, Mn, Nb, Ta, Ni, Cu, Co, Fe, Pd, Pt, Ag or combinations thereof;

Z is P, B, Si, Ge, C or combinations thereof;

a is between 2 and 45 at %; and

b is between 15 and 25 at %.

In yet another embodiment, the alloy or metallic glass is represented by the following formula:

X_(100-a-b)Y_(a)Z_(b),  Eq. (2)

where:

X is Ni, Fe, Co or combinations thereof

Y is Cr, Mo, Mn, Nb, Ta or combinations thereof

Z is P, B, Si, Ge or combinations thereof

a is between 5 and 15 at %

b is between 15 and 25 at %.

In yet another embodiment, the alloy melt is heated by a process that may include inductive heating, resistively heating (in a furnace), a plasma arc heating, or joule heating, where the melt is held in a crucible made of fused or crystalline silica, a ceramic such as alumina or zirconia, graphite, or a water-cooled hearth made of copper or silver.

In yet another embodiment, the alloy melt is quenched by a process that may include quenching the crucible containing the melt in a bath of room temperature water, iced water, or oil. The crucible is made of any of the aforementioned materials. Alternatively, the method includes quenching the melt by driving the melt under pressure or pouring the melt into a metal mold. In some embodiments, the mold is made of copper, brass, or steel.

Additional embodiments and features are set forth in part in the description that follows, and will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed subject matter. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to the following figures and data graphs, which are presented as various embodiments of the disclosure and should not be construed as a complete recitation of the scope of the disclosure, wherein:

FIG. 1 provides a schematic process profile to achieve improved glass-forming ability and toughness in alloys and metallic glasses in accordance with embodiments of the present disclosure.

FIG. 2 provides a plot showing the effect of melt overheating on the critical rod diameter and notch toughness of alloy and metallic glass Ni₆₉Cr_(8.5)Nb₃P_(16.5)B₃ in accordance with embodiments of the present disclosure.

FIG. 3 provides a plot showing the effect of melt overheating on the critical rod diameter and notch toughness of alloy and metallic glass Ni_(72.5)Cr₅Nb₃P_(16.5)B₃ in accordance with embodiments of the present disclosure.

FIG. 4 provides a plot showing the effect of melt overheating on the critical rod diameter and notch toughness of alloy and metallic glass Ni_(68.6)Cr_(8.7)Nb₃P₁₆B_(3.2)Si_(0.5) in accordance with embodiments of the present disclosure.

FIG. 5 provides a plot showing the effect of melt overheating on the critical rod diameter and notch toughness of alloy and metallic glass Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃ in accordance with embodiments of the present disclosure.

FIG. 6 provides a plot showing the effect of melt overheating on the critical rod diameter and notch toughness of alloy and metallic glass Fe₆₇Mo₆Ni_(3.5)Cr_(3.5)P₁₂C_(5.5)B_(2.5) in accordance with embodiments of the present disclosure.

FIG. 7 provides a plot showing the effect of cooling to an intermediate temperature after overheating the melt and prior to quenching on the critical rod diameter and notch toughness of alloy and metallic glass Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃ in accordance with embodiments of the present disclosure.

FIG. 8 provides a plot showing the effect of time spent at an intermediate temperature after overheating the melt and prior to quenching on the critical rod diameter and notch toughness of alloy and metallic glass Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃ in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure may be understood by reference to the following detailed description, taken in conjunction with the drawings as described below. It is noted that, for purposes of illustrative clarity, certain elements in various drawings may not be drawn to scale.

Description of the Processing Methods

The present disclosure provides methods of forming bulk metallic glasses or shaped metallic glass articles of improved toughness and glass forming ability by overheating the alloy melt to a temperature higher than T_(tough) and T_(GFA), which are both above T_(liquidus), prior to quenching. The present disclosure also provides an alternative method, whereas after overheating the alloy melt to above T_(tough) and T_(GFA), the melt is cooled and equilibrated to an intermediate temperature below both T_(tough) and T_(GFA) but above T_(liquidus), and subsequently quenched at a high enough rate to form a bulk metallic glass.

In the context of the present disclosure, glass-forming ability is understood as measured by the “critical rod diameter” as defined herein. Where the disclosure refers to improved or increased glass-forming ability, it will be understood to be as measured by the “critical rod diameter.” In the context of the present disclosure, toughness is measured by “notch toughness” as defined herein. Where the disclosure refers to improved or increased toughness, it will be understood to be as measured by the “notch toughness.”

In support of the former method, the present disclosure demonstrates that once the melt is heated to a temperature higher than T_(GFA), the glass forming ability of the alloy is considerably higher as compared to heating the melt just above T_(liquidus). The disclosure further demonstrates that once the melt is heated to a temperature higher than T_(tough), the toughness of the metallic glass is considerably higher as compared to heating the melt just above T_(liquidus).

In some embodiments, “heating the melt just above T_(liquidus)” or “in the absence of overheating” is intended to imply that the melt is overheated by less than 50° C. above the alloy liquidus temperature. In other embodiments, “just above T_(liquidus)” or “in the absence of overheating” is intended to imply that the melt is overheated by less than 5% of the alloy liquidus temperature (where T_(liquidus) is expressed in units of Kelvin).

In support of the latter method, the present disclosure demonstrates that once the melt is heated to a temperature higher than T_(GFA) and then annealed below T_(GFA) prior to quenching to form a glass, its higher glass-forming ability is actually retained. More surprisingly, the present disclosure reveals that once the melt is heated to a temperature higher than T_(tough) and then annealed at an intermediate temperature below T_(tough) prior to quenching to form a glass, its higher toughness is also retained.

The behavior of amorphous metals that are processed in the high-temperature melt state is complex. Quite unexpectedly, an independent threshold temperature above the liquidus temperature T_(liquidus) is identified to be associated with enhanced toughness, and is referred to as a first threshold temperature L_(tough) hereafter. Specifically, the metallic glasses processed by overheating the alloy melt above this threshold temperature, T_(tough), have an improved toughness at room temperature over the metallic glasses processed by heating the alloy melt above T_(liquidus) but below T_(tough).

Specifically, T_(tough) is defined as the melt overheating temperature associated with a substantial improvement in toughness, as measured by notch toughness, of the metallic glass at room temperature as compared to the toughness demonstrated in the absence of overheating above T_(liquidus), More specifically, T_(tough) may be identified as the temperature following the steepest increase in toughness with increasing melt overheating temperature (i.e. the temperature following the largest slope of the toughness function against temperature above T_(liquidus)).

Another threshold temperature, T_(GFA), is defined as the melt overheating temperature associated with substantial improvement in glass-forming ability, as measured by critical rod diameter, as compared to the glass-forming ability demonstrated in the absence of overheating above T_(liquidus). More specifically, T_(GFA) may be identified as the temperature following the steepest increase in glass forming ability with increasing melt overheating temperature (i.e. the temperature following the largest slope of the glass forming ability function against temperature above T_(liquidus)).

In some embodiments, “significant improvement” in toughness and glass-forming ability may be interpreted as an improvement of at least 10% compared to the respective values obtained in the absence of overheating above T_(liquidus). In some embodiments, “substantial improvement” in toughness and glass-forming ability may be interpreted as an improvement of at least 25% compared to the respective values obtained in the absence of overheating above T_(liquidus). In some embodiments, “substantial improvement” is interpreted as an improvement of at least 50% compared to the respective values obtained in the absence of overheating above T_(liquidus). In some embodiments, “substantial improvement” is interpreted as an improvement of at least 75% compared to the respective values obtained in the absence of overheating above T_(liquidus).

In some embodiments, “substantial improvement” in toughness and glass-forming ability may be interpreted as an improvement of at least 50% compared to the respective values obtained in the absence of overheating above T_(liquidus) attained by overheating the melt by at least 100° C. above T_(liquidus). In some embodiments, “substantial improvement” is interpreted as an improvement of at least 50% compared to the respective values obtained in the absence of overheating above T_(liquidus) attained by overheating the melt by at least 50° C. above T_(liquidus). In some embodiments, “substantial improvement” is interpreted as an improvement of at least 50% compared to the respective values obtained in the absence of overheating above T_(liquidus) attained by overheating the melt by at least 25° C. above T_(liquidus).

FIG. 1 provides a schematic process profile to achieve improved glass-forming ability and toughness in alloys and metallic glasses in accordance with embodiments of the present disclosure. As shown in FIG. 1, one processing path 102 includes passing through the identified threshold temperatures associated with improved toughness, T_(tough), and with improved glass-forming ability, T_(GFA), followed by quenching from an overheating temperature above T_(tough), like for example from the designated temperature T_(H).

Both T_(tough) and T_(GFA) may be higher than the solidus and liquidus temperatures T_(solidus) and T_(liquidus), respectively. In some embodiments, T_(tough) may be higher than T_(GFA), as shown in FIG. 1. Although this temperature order is not limiting, most metallic glass systems may have T_(tough) higher than T_(GFA). In other embodiments, T_(tough) may be lower than T_(GFA).

Regardless of the order of T_(tough) versus T_(GFA), it is observed that T_(tough) is independent of and different from T_(GFA). Accordingly, attempts to achieve high glass-forming ability by overheating the melt above a certain T_(GFA) would not necessarily lead to a tougher metallic glass.

As shown in FIG. 1, in order to achieve high glass-forming ability, the melt can be heated to a temperature higher than T_(GFA) prior to quenching. Furthermore, the melt can be heated to a temperature higher than both T_(GFA) and T_(tough) prior to quenching in order to achieve both a high glass-forming ability and a high toughness.

An alloy would not only have improved glass-forming ability, i.e. being capable of forming a metallic glass in larger lateral dimensions, but would also form a metallic glass article or hardware having an improved toughness according to the present overheating method. One of the benefits of the improved toughness is to enable the metallic glass article or hardware formed from the alloy to evade catastrophic fracture upon loading initiating from structural flaws, particularly in bending loading. Specifically, an amorphous metal article having a lateral dimension of at least 0.5 mm made according to the present method would be able to undergo macroscopic plastic bending when overloaded, evading catastrophic fracture. This improved toughness, together with the improved glass-forming ability, can result in an improved overall engineering applicability and performance.

The present disclosure also provides a method for the melt to retain “memory” of its high temperature state at intermediate temperatures. Specifically, after heating the melt to a temperature higher than both T_(GFA) and T_(tough), the melt may be cooled to an intermediate temperature below T_(GFA) and T_(tough) but above T_(liquidus) prior to quenching, and may be held isothermally at the intermediate temperature without substantially losing the gains in both the glass-forming ability and toughness attained by initially overheating to above T_(GFA) and T_(tough). FIG. 1 illustrates an alternative path 104 involving the isothermal step at an intermediate temperature which is lower than both T_(GFA) and T_(tough) but higher than T_(liquidus), like for example at the designated temperature T_(L)

One of the benefits of the alternative path is to limit the degradation of a metal mold tool by avoiding injecting the melt into the mold tool from very high temperatures. For processes that utilize pressure to drive the processed melt into a metal mold in order to shape the melt and simultaneously quench the melt to form an amorphous metal article or hardware, such as die casting, the ability of the melt to retain “memory” of its high temperature state is very important. This is because the tool life of the mold depends strongly on the temperature of the alloy melt. To achieve the high toughness and high glass-forming ability, the alloy is overheated to be above T_(GFA) and T_(tough) which may be much higher than T_(liquidus). With such high temperatures, the tool life of mold may be dramatically shortened using the present method at a temperature above T_(GFA) and T_(tough), like for example at T_(H), as shown in processing path 102. However, injecting the melt into the mold at a lower intermediate temperature below T_(GFA) and T_(rough) but above T_(liquidus), like for example at T_(L), according to the alternative processing path 104, would diminish any adverse effects on the tool life. Other potential benefits include lower power requirements for heating the melt, less thermal shrinkage of the part, and potentially better melt flow control with higher viscosity at lower temperature.

The present method is applicable to any processing that produces an amorphous metal article or part by melting and quenching a metallic alloy.

The method is also applicable, without limitation, to any heating process that involve melting the alloy. Heating processes may include, without limitation, inductive heating, resistive heating (e.g. in a furnace), plasma arc heating, or joule heating, where the alloy melt is held in a crucible. The crucible material may include, without limitation, fused or crystalline silica, a ceramic such as alumina or zirconia, graphite, or a water-cooled hearth made of copper or silver.

The method is also applicable, without limitation, to any quenching processes that involve quenching the crucible containing the melt in a bath of room temperature water, iced water, or oil, or quenching the melt by driving it under pressure or pouring it into a metal mold made of copper, brass, or steel. The crucible may be made of any of the aforementioned materials.

The disclosed methods are applicable to all metal alloys capable of forming a metallic glass by quenching the alloy melt form high temperature. A “critical cooling rate”, which is defined as the cooling rate required to avoid crystallization and form the amorphous phase of the alloy (i.e. the metallic glass) determines the critical rod diameter. The lower the critical cooling rate of an alloy, the larger its critical rod diameter. The critical cooling rate R_(c) in K/s and critical rod diameter d_(c) in mm are related via the following approximate empirical formula:

R _(c)=1000/d _(c) ²  Eq. (2)

According to Eq. (2), the critical cooling rate for an alloy having a critical rod diameter of about 1 mm, as in the case of the alloys according to embodiments of the present disclosure, is only about 10³ K/s.

Generally, three categories are known in the art for identifying the ability of a metal alloy to form glass (i.e. to bypass the stable crystal phase and form an amorphous phase). Metal alloys having critical cooling rates in excess of 10¹² K/s are typically referred to as non-glass formers, as it is physically impossible to achieve such cooling rates over a meaningful thickness. Metal alloys having critical cooling rates in the range of 10⁵ to 10¹² K/s are typically referred to as marginal glass formers, as they are able to form glass over thicknesses ranging from 1 to 100 micrometers according to Eq. (2). Metal alloys having critical cooling rates on the order of 10³ or less, and as low as 1 or 0.1 K/s, are typically referred to as bulk glass formers, as they are able to form glass over thicknesses ranging from 1 millimeter to several centimeters. The glass-forming ability of a metallic alloy is, to a very large extent, dependent on the combination and composition of the alloy. It is important to state that the combinational and compositional ranges for alloys capable of forming marginal glass formers are considerably broader than those for forming bulk glass formers.

The present method is applicable to any metallic glass-forming alloy, including but not limited to, Zr-based, Ti-based, Al-based, Mg-based, Ce-based, La-based, Ca-based, Y-based, Fe-based, Ni-based, Co-based, Cu-based, Au-based, Pd-based, and Pt-based.

Without limitation, Zr-based glass-forming alloys may include elements selected from the group consisting of Ti, Ni, Cu, Be, Hf, Nb, V, Al, Sn, Ag, Pd, Fe, Co, and Cr.

Without limitation, Fe-based glass-forming alloys may include elements selected from the group consisting of Co, Ni, Mo, Cr, P, C, B, Si, Al, Zr, W, Mn, Y, and Er.

Without limitation, Ni-based glass-forming alloys may include elements selected from the group consisting of Co, Fe, Cu, Mo, Cr, P, B, Si, Sn, Nb, Ta, V, and Mn.

Without limitation, Cu-based glass-forming alloys may include elements selected from the group consisting of Zr, Ti, Ni, Au, Ag, Hf, Nb, V, Si, Sn, and P.

Without limitation, Au-based glass-forming alloys may include elements selected from the group consisting of Cu, Si, Ag, Pd, Pt, Ge, Y, and Al.

Without limitation, Pd-based glass-forming alloys may include elements selected from the group consisting of Pt, Ni, Cu, P, Si, Ge, Ag, Au, Fe, and Co.

Without limitation, Pt-based glass-forming alloys may include elements selected from the group consisting of Pd, Ni, Cu, P, Si, Ge, Ag, Au, Fe, and Co.

In some embodiments, for certain alloys whose melt can be fluxed to increase glass-forming ability, fluxing can also help achieve both high toughness and high glass-forming ability without the need for melt overheating. A fluxing method is disclosed in a recent patent application U.S. Patent No. 61/913,732, filed on Dec. 9, 2013, entitled “Melt fluxing method for improved toughness and glass-forming ability of metallic glasses and glass forming alloys”, which is incorporated herein by reference in its entirety. Fluxing the alloy melt may help avoid overheating to very high temperatures in order to achieve high toughness and high glass-forming ability.

EXAMPLES

The following non-limiting examples are illustrative of aspects of the present disclosure.

Example 1 Melt Overheating to a Temperature Above T_(tough) and T_(GFA)

To demonstrate the effects of the method of melt overheating at T_(H) on glass-forming ability (GFA) and toughness, Ni-based glass-forming alloys from the Ni—Cr—Nb—P—B family, disclosed in a recent application (U.S. Patent Application No. 61/720,015, entitled “Bulk Nickel-Based Chromium and Phosphorous Bearing Metallic Glasses with High Toughness”, filed on Oct. 30, 2012, which is incorporated herein by reference), and the Fe-based glass forming alloy Fe₆₇Mo₆Ni_(3.5)Cr_(3.5)P₁₂C_(5.5)B_(2.5) are used here as example systems.

The glass-forming ability of each alloy was assessed by determining the “critical” rod diameter”, defined as the maximum rod diameter at which the amorphous phase can be formed when processed by the method of water quenching the molten alloy in quartz tubes having 0.5 mm wall thickness.

FIG. 2 provides a plot showing the effect of melt overheating on the glass-forming ability and toughness of alloy and metallic glass Ni₆₉Cr_(8.5)Nb₃P_(16.5)B₃ in accordance with embodiments of the present disclosure. As presented in FIG. 2, the glass-forming ability is shown to improve if the melt is heated to above its T_(GFA) of 1100° C. or higher. For example, when the melt is heated to 1050° C., which is below T_(GFA), and subsequently quenched, the alloy is found to have a critical rod diameter of about 3 mm. In contrast, when the melt is heated to 1250° C., which is substantially higher than T_(GFA), and subsequently quenched, the alloy yields a substantially improved glass-forming ability, i.e. a critical rod diameter of 10 mm.

However, the alloy with improved glass-forming ability still lacks good toughness when heated to 1250° C., showing a room-temperature notch toughness of just 30 MPa m^(1/2). Surprisingly, when heating the alloy to 1350° C., which is above its T_(tough) of 1300° C., and subsequently quenching, the alloy forms a metallic glass that has a substantially improved toughness of about 80 MPa m^(1/2).

The same effect is shown for four more alloys. For each alloy, one can define values for T_(GFA) and T_(tough). FIG. 3 provides a plot showing the effect of melt overheating on the glass-forming ability and toughness of alloy and metallic glass Ni_(72.5)Cr₅Nb₃P_(16.5)B₃ in accordance with embodiments of the present disclosure. As shown in FIG. 3, alloy Ni_(72.5)Cr₅Nb₃P_(16.5)B₃ has a T_(GFA) of 1100° C. and T_(tough) of 1150° C.

FIG. 4 provides a plot showing the effect of melt overheating on the glass-forming ability and toughness of alloy and metallic glass Ni_(68.6)Cr_(8.7)Nb₃P₁₆B_(3.2)Si_(0.5) in accordance with embodiments of the present disclosure. As shown in FIG. 4, the Ni_(68.6)Cr_(8.7)Nb₃P₁₆B_(3.2)Si_(0.5) alloy has a T_(GFA) of 1150° C. and T_(tough) of 1250° C.

FIG. 5 provides a plot showing the effect of melt overheating on the glass-forming ability and toughness of alloy and metallic glass Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃ in accordance with embodiments of the present disclosure. As shown in FIG. 5, the Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(3.46)B₃ alloy has a T_(GFA) of 1125° C. and T_(tough) of 1250° C.

FIG. 6 provides a plot showing the effect of melt overheating on the glass-forming ability and toughness of alloy and metallic glass Fe₆₇Mo₆Ni_(3.5)Cr_(3.5)P₁₂C_(5.5)B_(2.5) in accordance with embodiments of the present disclosure. As shown in FIG. 6, the Fe₆₇Mo₆Ni_(3.5)Cr_(3.5)P₁₂C_(5.5)B_(2.5) alloy has a T_(GFA) of 1350° C. and T_(tough) of 1450° C.

The alloy and metallic glass compositions along with the associated T_(liquidus), T_(GFA), and T_(tough) values in degrees Celcius (° C.) are presented in Table 1. As shown, for this alloy family T_(tough) is higher than T_(GFA) in all four compositions in Table 1, and both T_(tough) and T_(GFA) are substantially higher than T_(liquidus). The degree of overheating to achieve the high glass-forming ability and toughness, respectively defined as ΔT_(GFA)=T_(GFA)−T_(liquidus) and ΔT_(tough)=T_(tough)−T_(liquidus), are also presented for each composition in Table 1.

TABLE 1 Values for T_(liquidus), T_(GFA), T_(tough), ΔT_(GFA) and ΔT_(tough) for sample alloys and metallic glasses (in degrees Celcius) Alloy composition T_(liquidus) T_(GFA) T_(tough) ΔT_(GFA) ΔT_(tough) Ni₆₉Cr_(8.5)Nb₃P_(16.5)B₃ 874° C. 1100° C. 1300° C. 226° C. 426° C. Ni_(72.5)Cr₅Nb₃P_(16.5)B₃ 882° C. 1100° C. 1150° C. 218° C. 268° C. Ni_(68.6)Cr_(8.7)Nb₃P₁₆B_(3.2)Si_(0.5) 884° C. 1150° C. 1250° C. 266° C. 366° C. Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃ 881° C. 1125° C. 1250° C. 244° C. 369° C. Fe₆₇Mo₆Ni_(3.5)Cr_(3.5)P₁₂C_(5.5)B_(2.5) 1030° C.  1350° C. 1400° C. 320° C. 370° C.

Table 2 presents values for T_(liquidus), T_(GFA), T_(tough), ratios of ΔT_(GFA)/T_(liquidus) and ΔT_(tough)/T_(liquidus) for sample alloys and metallic glasses (in degrees Kelvin).

TABLE 2 Values for T_(liquidus), T_(GFA), T_(tough), ratios of ΔT_(GFA)/T_(liquidus) and ΔT_(tough)/T_(liquidus) for sample alloys and metallic glasses (in degrees Kelvin) Alloy composition T_(liquidus) T_(GFA) T_(tough) ΔT_(GFA)/T_(liquidus) ΔT_(tough)/T_(liquidus) Ni₆₉Cr_(8.5)Nb₃P_(16.5)B₃ 1147 K 1373 K 1573 K 0.197 0.371 Ni_(72.5)Cr₅Nb₃P_(16.5)B₃ 1155 K 1373 K 1423 K 0.189 0.232 Ni_(68.6)Cr_(8.7)Nb₃P₁₆B_(3.2)Si_(0.5) 1157 K 1423 K 1523 K 0.230 0.316 Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃ 1154 K 1398 K 1523 K 0.211 0.320 Fe₆₇Mo₆Ni_(3.5)Cr_(3.5)P₁₂C_(5.5)B_(2.5) 1303 K 1623 K 1673 K 0.246 0.284

Example 2 Melt Overheating to a Temperature Above T_(tough) and T_(GFA) and Subsequently Cooling to an Intermediate Temperature Below T_(tough) and T_(GFA) but Above T_(Liquidus)

The effects of overheating the melt to a temperature above T_(tough) and T_(GFA) and subsequently cooling to an intermediate temperature below T_(tough) and T_(GFA) but above T_(liquidus) on glass-forming ability and toughness is investigated for alloy Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃. The alloy is melted and the melt is overheated to a temperature at least as high as T_(tough) and T_(GFA), followed by cooling to an intermediate temperature below T_(tough) and T_(GFA) but above T_(liquidus) for a fixed period of time, and then quenched to form a metallic glass.

FIG. 7 provides a plot showing the effect of cooling to an intermediate temperature after overheating the melt and prior to quenching on the glass-forming ability and toughness of alloy and metallic glass Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃ in accordance with embodiments of the present disclosure. The critical rod diameters and notch toughness of the metallic glasses are plotted in FIG. 7.

As shown in FIG. 7, when the melt is heated to 1250° C., which is higher than its T_(GFA) of 1125° C. and equal to T_(tough) of 1250° C., and then subsequently quenched, the critical rod diameter is about 11 mm and the toughness about 85 MPa m^(1/2).

Also, when the melt is heated to 1250° C., subsequently annealed at an intermediate temperature of 1100° C., which is slightly below its T_(GFA) of 1125° C. and well below its T_(tough) of 1250° C., and then quenched, the critical rod diameter drops slightly to about 9 mm and the toughness drops slightly to about 70 MPa m^(1/2).

Furthermore, when the melt is heated to 1250° C., subsequently annealed at a lower intermediate temperature of 950° C., which is substantially below both its T_(GFA) and T_(tough), and then quenched, the critical rod diameter remains high at about 9 mm but the toughness drops sharply to about 30 MPa m^(1/2).

When the melt is annealed at lower intermediate temperatures below 950° C. (e.g. 900° C.) and above T_(liquidus) after first being heated to 1250° C., both the toughness and the critical rod diameter appear to also drop sharply to about 30 MPa m^(1/2) and 7 mm, respectively.

Hence, these results suggest that although the threshold temperatures T_(GFA) and T_(tough) are quite high, one does not have to quench from such high temperature and encounter the associated adverse effects (e.g. low tool life in the case of die casting) in order to improve or enhance glass-forming ability and toughness. Rather, one can cool the melt to an intermediate temperature (e.g. by transferring it to another cooler reservoir), such as 1100° C. for Ni_(71.4)Cr_(5.64)Nb_(3.64)P_(16.5)B₃ rather than 1250° C., and then quench from that intermediate temperature, thus avoiding the adverse high-temperature effects while retaining significant glass-forming ability and toughness.

The effect of time on holding the melt isothermally at an intermediate temperature below T_(GFA) and T_(tough) but above T_(liquidus) following overheating above T_(GFA) and T_(tough) is also investigated for alloy Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃. The alloy is melted and the melt is overheated to a temperature of at least as high as T_(tough) and T_(GFA), followed by cooling to an intermediate temperature below T_(GFA) and T_(tough) but above T_(liquidus) where it is held for various periods of time, and then quenched to form the metallic glass.

FIG. 8 provides a plot showing the effect of time spent at an intermediate temperature after overheating the melt and prior to quenching on the glass-forming ability and toughness of alloy and metallic glass Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃ in accordance with embodiments of the present disclosure.

As shown in FIG. 8, the melt is first heated to 1250° C. and held there for about 180 seconds, then cooled and allowed just enough time to equilibrate to an intermediate temperature of 1150° C. The alloy melt is quenched immediately after equilibration at 1150° C., and the critical rod diameter of the Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃ alloy is found to be about 9 mm and the toughness of the Ni_(71.4)Cr_(5.64)Nb_(3.46)P_(16.5)B₃ metallic glass about 85 MPa m^(1/2).

When the alloy melt is held for about 450 seconds and also for about 900 seconds at 1150° C. in addition to the time required to equilibrate to that temperature and then subsequently quenched, the critical rod diameter and toughness remain essentially unchanged.

This result reveals that no transient processes take place during isothermal holding at the intermediate temperature. This result suggests that after the melt is heated to above T_(GFA) and T_(tough), the melt can be cooled to an intermediate temperature below T_(GFA) and T_(tough) but above T_(liquidus) and held there for a long period of time without affecting the enhanced glass-forming ability and toughness.

Description of Methods of Investigating the Melt Overheating Effect on Toughness and GFA

A particular method for producing the example alloys of the present disclosure involves inductive melting of the appropriate amounts of elemental constituents in a fused silica crucible under inert atmosphere to form alloy ingots. Alternatively, the crucible may also be crystalline silica, a ceramic such as alumina or zirconia, graphite, or a water-cooled hearth made of copper or silver. Particular purity levels of the constituent elements were as follows: Ni 99.995%, Cr 99.996% (crystalline), Nb 99.95%, B 99.5%, Si 99.9999, and P 99.9999%, Fe 99.95%, Mo 99.95%, and C 99.9995%.

A particular method for producing metallic glass rods from the alloys of the present disclosure involves re-melting the alloy ingots in quartz tubes having 0.5 mm thick walls in a furnace under high purity argon. After processing at specific temperatures, the melt is rapidly quenching in a room-temperature water bath.

In various experiments, the melt is heated to an overheating temperature above the liquidus temperature, followed by quenching to form metallic glass rods. The critical rod diameter of the alloys associated with the specific overheating temperature was determined. Another 3-mm diameter rod was produced for each overheating temperature following the same procedure, and the toughness of the 3-mm diameter metallic glass rod was measured. These data are presented in FIGS. 2-6.

In various experiments, the melt is first heated to an overheating temperature, followed by cooling to an intermediate temperature, and after equilibrating at the intermediate temperature then quenched. The critical rod diameter of the alloys associated with the specific overheating and intermediate temperature was determined. Another 3-mm diameter rod was produced for each overheating and intermediate temperature following the same procedure, and the toughness of the 3-mm diameter metallic glass rod was measured. These data are presented in FIG. 7.

In various experiments, the melt is first heated to an overheating temperature, followed by cooling to an intermediate temperature, and after equilibrating at the intermediate temperature it was held there for a specific period of time, and then quenched. The critical rod diameter of the alloys associated with the specific overheating and intermediate temperatures and specific period of time was determined. Another 3-mm diameter rod was produced for each overheating and intermediate temperature and specific period of time following the same procedure, and the toughness of the 3-mm diameter metallic glass rod was measured. These data are presented in FIG. 8.

Test Methodology for Assessing Glass Forming Ability

The glass-forming ability of each alloy was assessed by determining the maximum rod diameter, i.e. “critical rod diameter”, in which the amorphous phase of the alloy (i.e. the metallic glass phase) could be formed when processed by the method of quenching the alloy melt contained in a quartz tube with 0.5 mm thick walls in a bath of room temperature water, as described above. X-ray diffraction with Cu-Kα radiation was performed to verify the amorphous structure of the alloys.

Test Methodology for Measuring Notch Toughness

Measurement of notch toughness of the example alloys was performed on 3-mm diameter amorphous rods at room temperature. The rods were notched using a wire saw with a root radius of between 0.10 and 0.13 μm to a depth of approximately half the rod diameter. The notched specimens were placed on a 3-point bending fixture with span distance of 12.7 mm and carefully aligned with the notched side facing downward. The critical fracture load was measured by applying a monotonically increasing load at constant cross-head speed of 0.001 mm/s using a screw-driven testing frame. At least three tests were performed, and the variance between tests is included in the notch toughness plots. The stress intensity factor for the geometrical configuration employed here was evaluated using the analysis by Murakimi (Y. Murakami, Stress Intensity Factors Handbook, Vol. 2, Oxford: Pergamon Press, p. 666 (1987)).

Having described several embodiments, it will be recognized by those skilled in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention.

Those skilled in the art will appreciate that the presently disclosed embodiments teach by way of example and not by limitation. Therefore, the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween. 

1. A method of forming a bulk metallic glass, comprising: melting an alloy by heating the alloy to a temperature above the liquidus temperature, T_(liquidus); overheating the alloy melt to an overheating temperature above a threshold temperature, T_(tough), associated with the metallic glass demonstrating increased toughness compared to the toughness demonstrated by heating the melt just above T_(liquidus) and quenching the melt to form a bulk metallic glass.
 2. The method of claim 1, wherein the temperature of the overheated alloy melt is also above a threshold temperature, T_(GFA), associated with the metallic glass demonstrating increased critical rod diameter compared to the critical rod diameter demonstrated by heating the melt just above T_(liquidus).
 3. The method of claim 2, wherein T_(tough) is greater than T_(GFA).
 4. The method of claim 1, wherein the toughness of the metallic glass is at least 25% greater than the toughness of the metallic glass formed in the absence of overheating above T_(liquidus).
 5. The method of claim 1, wherein the toughness of the metallic glass is at least 50% greater than the toughness of the metallic glass formed in the absence of overheating above T_(liquidus).
 6. The method of claim 2, wherein the critical rod diameter is at least 25% greater than the critical rod diameter attained in the absence of overheating above T_(liquidus).
 7. The method of claim 2, wherein the critical rod diameter is at least 50% greater than the critical rod diameter attained in the absence of overheating above T_(liquidus).
 8. The method of claim 1, further comprising cooling the alloy melt to an intermediate temperature below T_(tough) and T_(GFA) but above T_(liquidus), equilibrating the alloy melt at the intermediate temperature, and quenching the alloy melt to form the metallic glass.
 9. The method of claim 1, wherein the alloy is selected from a Zr-based alloy, Ti-based alloy, Al-based alloy, Mg-based alloy, Ce-based alloy, La-based alloy, Y-based alloy, Fe-based alloy, Ni-based alloy, Co-based alloy, Cu-based alloy, Au-based alloy, Pd-based alloy, and Pt-based alloy.
 10. The method of claim 1, wherein the alloy is represented by the formula X_(100-a-b)Y_(a)Z_(b) where: X is Ni, Fe, Co, Pd, Pt, Au, Cu or combinations thereof; Y is Cr, Mo, Mn, Nb, Ta, Ni, Cu, Co, Fe, Pd, Pt, Ag or combinations thereof; Z is P, B, Si, Ge, C or combinations thereof; a is between 2 and 45 at %; and b is between 15 and 25 at %.
 11. The method of claim 1, wherein the alloy is represented by the formula X_(100-a-b)Y_(a)Z_(b), wherein: X is Ni, Fe, Co or combinations thereof, Y is Cr, Mo, Mn, Nb, Ta or combinations thereof, Z is P, B, Si, Ge or combinations thereof, a is between 5 and 15 at %, and b is between 15 and 25 at %.
 12. The method of claim 1, wherein the alloy melt is heated by a process selected from inductive heating, resistively heating (in a furnace), a plasma arc heating, and joule heating.
 13. The method of claim 1, wherein the melt is held in a crucible comprising a material selected from fused or crystalline silica, a ceramic, alumina, zirconia, graphite, and a water-cooled hearth made of copper or silver.
 14. A method of forming a shaped metallic glass article, comprising: melting a metallic glass forming alloy by heating the alloy to a temperature above the liquidus temperature of the alloy, T_(liquidus). overheating the alloy melt to an overheating temperature above both a threshold temperature, T_(tough), associated with the metallic glass demonstrating increased toughness compared to the toughness demonstrated by heating the melt just above T_(liquidus), and a threshold temperature, T_(GFA), associated with the alloy demonstrating an increase in critical rod diameter compared to the critical rod diameter demonstrated by heating the melt just above T_(liquidus); and quenching the alloy melt to form the alloy melt into a shaped metallic glass article.
 15. The method of claim 14, further comprising cooling and equilibrating the alloy melt to an intermediate temperature below T_(tough) and T_(GFA) but above T_(liquidus); and quenching the alloy melt to form a shaped metallic glass article.
 16. The method of claim 14, wherein the toughness of the metallic glass is at least 25% greater than the toughness of the metallic glass formed in the absence of overheating above T_(liquidus).
 17. The method of claim 14, wherein the toughness of the metallic glass is at least 50% greater than the toughness of the metallic glass formed in the absence of overheating above T_(liquidus).
 18. The method of claim 14, wherein the critical rod diameter is at least 25% greater than the critical rod diameter attained in the absence of overheating above T_(liquidus).
 19. The method of claim 14, wherein the critical rod diameter is at least 50% greater than the critical rod diameter attained in the absence of overheating above T_(liquidus).
 20. The method of claim 14, wherein the alloy comprises a material selected from a group consisting of a Zr-based alloy, Ti-based alloy, Al-based alloy, Mg-based alloy, Ce-based alloy, La-based alloy, Y-based alloy, Fe-based alloy, Ni-based alloy, Co-based alloy, Cu-based alloy, Au-based alloy, Pd-based alloy, and Pt-based alloy. 